A 64-nuclearity silver cluster encapsulating a unique POM anion [MnIIIMnIV2Mo14O56]17– has been synthesized. The formation of the templating core performs a reassembly process for increasing nuclearities from {MnMo9} to {Mn3Mo14}. It represents a rare inorganic meso anion containing mixed-valent Mn that is built up by d-{MnIVMo7} and l-{MnIVMo7} units connecting together through a {MnIII} fragment.